The invention relates to controlling contrast in an optical projection system which uses a display cell.
Unlike an overhead projector which uses a transparency or slide to generate an image (e.g., a figure or photograph) on a screen, a typical liquid crystal projector generates the image via a liquid crystal display cell. The display cell is electrically controlled which allows, for example, a portable computer (interfaced to the display cell) to generate and revise the image during a presentation.
The display cell may be reflective or transmissive. If transmissive, light used for illumination passes through the display cell to project the image upon the screen. If the display cell is reflective, the light is selectively reflected off a back surface of the display cell to project the image on the screen.
Both display cells take advantage of the fact that light may either be scattered, absorbed, or reflected by subregions of the display cell, called pixels, to form the image. Each pixel is electrically placed in either a scattering state or a clear state. In the scattering state, the pixel scatters and/or absorbs the incident illumination light and appears dark in color. In the clear state, the pixel reflects the incident light (for a reflective display cell) or allows the incident light to pass (for a transmissive display cell) to appear bright in color.
The light used for illumination typically is generated by an arc lamp which is located near a parabolic mirror. The mirror collimates the light generated by the lamp and directs the collimated light toward the display cell. A portion of the collimated light is intercepted by the glass arc tube of the arc lamp which creates a dark spot on the display cell. To remove the dark spot, the projector may pass the collimated light (with the dark spot) through an optical integrator to average the illumination distribution of the light and thus, eliminate the dark spot.
The perceived contrast ratio of the image formed on the screen may be defined as the ratio of the brightest spot of the image to the darkest spot of the image, and the perceived contrast ratio is a function of the brightness of the room (where the image is being viewed) and the f-number (a measure of the amount of light introduced into the image by the projector) of the projector. The effect of the f-number is particularly important for projectors that utilize scattering to generate images or where unintentional scattering occurs. However, the perceived contrast ratio may be different than an actual contrast ratio (the contrast ratio of the image coming out of the projector) set by the projector due to the contribution of ambient light present in the viewing environment.
Accordingly, it would be useful if the contrast ratio of the projector could be controlled. The present invention provides a system and method that provides such control.